AUTUMN, K.: The Gecko Effect: Dynamic Dry Adhesive Microstructures

In the 4th century B.C.E., Aristotle extolled the ability of the gecko to, �run up and down a tree in any way, even with the head downwards.� Two millennia later, we have discovered many of the secrets of gecko adhesion, yet the millions of dry, adhesive setae on the toes of geckos continue to produce new questions and valuable answers. Each seta ends in hundreds of 200 nm spatular tips, permitting intimate contact with rough and smooth surfaces alike. Our latest results support the hypothesis that setae adhere by weak intermolecular forces. We provided direct support for the van der Waals hypothesis of gecko adhesion, and rejected surface polarity as a predictor of adhesion force, as suggested previously. Recently, we showed that, while van der Waals dry adhesion occurs at the level of individual setae, the integration of thousands of setae in an array yields complex -even fluid-like- dynamics that may enable a gecko to maintain adhesion during large, rapid perturbations. Previously, my collaborators and I measured the force production of a single seta. Initial efforts to attach a seta failed because of improper 3D orientation. However, by simulating the dynamics of gecko legs during climbing (based on force plate data) we discovered that a small normal preload, combined with a 5&microm displacement yielded a very large adhesive force of 200&microN, 10 times that predicted by whole-animal measurements. Six million setae of a single Tokay gecko attached maximally could generate 120kg force. This raises the question of how geckos manage to detach their feet in just 15ms. We discovered that simply increasing the angle that the setal shaft makes with the substrate to 30&deg causes detachment. Understanding how simultaneous attachment and release of millions of setae are controlled will require an approach that integrates levels ranging from molecules to lizards.